This invention relates to fastener tightening systems for gasketed joints. More specifically it relates to a torque applying system having tension related feedback for controlling and monitoring the tightening of a threaded fastener joint which includes a gasket.
When pressure is applied to a gasket by a fastener such as a bolt tightened to a specific torque, the gasket tends to compress under pressure. The compression of the gasket under pressure relieves the clamping force on the gasket supplied by the fastener. This can result in the loss of gasket sealing and/or loose fasteners after a short period of time.
A gasket of a given material has a rate of compression that is a function of the applied pressure. Gaskets are typically used as seals between two surfaces that are secured to each other by fasteners such as bolts. In presently known systems, fasteners are tightened to a specific torque or rotated to a specified angle of turn thereby causing pressure to be applied to the gasket. When a specific torque or a specified angle is reached the fastener driving device is turned off. However, even though the fastener tightening process is terminated, the gasket will continue to compress until the gasket's resistance to flow under pressure equals the pressure produced by the tensioned fastener. The net result is that over a short period of time, the tension originally applied by a fastener may be reduced substantially, often to 60% or less of the original tension value. This loss of fastener tension in present known tightening systems results in loose fasteners and ineffective gasket seals.
Various tightening systems and methods have been employed to overcome the undesirable effects of gasket relaxation. A first method is the sustained power method. This method tightens a fastener to a selected torque limit and then maintains power on the drive system at a level sufficient to maintain the torque level over a predetermined period of time. The method achieves some success because as the gasket flows and fastener tension relaxes, the drive system attempts to turn the fastener in order to hold fastener tension at the desired level.
Another method is the power ramp method. Slowly increasing torque is applied to the fastener up to the desired torque level. The fastener will turn to follow the compression rate of the gasket.
Another method is the cyclic torque pulse method as described in U.S. Pat. No. 3,886,822 for example. An initial torque at a level somewhat below the desired final torque is continuously applied. A series of torque applications whose peaks are approximately equal to the desired final torque are then applied.
Another method is the torque pulse method as recently described in U.S. patent application Ser. No. 048,442 filed May 11, 1987 titled "Gasketed Joint Tightening Means and Method" and assigned to the same assignee as the present invention. The torque pulse method applies a user selected number of equal amplitude torque pulses to the gasketed joint each followed by a time pause of substantially reduced power. The system provides for ongoing compensation for gasket compression by repeatedly tightening the fastener using a predefined torque shutoff level. Control is provided by controlling the number of torque pulses or the total elapsed time in which the pulses are applied.
The major disadvantage of all the gasket tightening strategies currently in use is the complete lack of feedback related to relaxation of the gasket being compressed. Present methods, with the exception of the torque pulse system noted above, use overall time as a basis for determining adequate tightening, i.e., they make the assumption that if a fixed amount of pressure is applied to a gasket for a fixed time period, the gasket will be adequately compressed. Overall time is not a reliable indicator of gasket compression. Most of these systems use torque as a control for gasket compression. Torque determines the clamping force applied to a gasket, but torque alone is not a reliable indicator of gasket compression, i.e., the gasket may still compress or flow after the torque application ceases.